Fix 1 error and 2 failures in unit tests under Python 3, e.g.:
```
FAIL: test_render (unit.visualizations.plugins.test_VisualizationPlugin.VisualizationsPlugin_TestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/travis/build/galaxyproject/galaxy/test/unit/visualizations/plugins/test_VisualizationPlugin.py", line 180, in test_render
self.assertIsInstance(response, string_types)
AssertionError: b'\nTrue\n' is not an instance of (<class 'str'>,)
```
- Add flake8-import-order to flake8 Pipfile and remove py27-lint-imports
and py27-lint-imports-include-list tox envs
- Fix most E201 and E202 errors reported by flake8-import-order v0.15,
but pin flake8-import-order to v0.14.3 until
https://github.com/PyCQA/flake8-import-order/issues/123
is fixed
This let us drop 2 jobs on Travis per each job.
not only for converting jupyter notebook to HTML (with nbconvert) but also
where ipython/IPython was referenced.
Some files still contain ipython but this is mostly related to documentation or
to the IE docker container itself.